The present invention relates to a process for the production of catalytic converters for purifying exhaust gases, and more particularly to a method for producing such catalytic converters by providing a pre-formed metal shell and supporting mat for a honeycomb catalyst or catalyst support, and compressing the supporting mat material against the metal shell prior to axially inserting the honeycomb catalyst therein.
As is well known, the purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is generally achieved by an exhaust gas purification system in which a ceramic element having a honeycomb cell structure acts as a catalyst carrier. Typically, this honeycomb cell structure is covered with a supporting washcoat and a catalyst that contains a precious metal which functions, in the presence of O2, to convert noxious components of the exhaust gas, such as HC and CO, to H2O and CO2. The honeycomb cell structure is housed within a gas-tight, sheet metal or cast metal heat resistant housing commonly referred to as a can or shell.
Honeycomb structures currently employed are typically comprised of a ceramic material such as cordierite, a brittle material exhibiting limited mechanical strength in very thin cross-section. For this reason, catalytic converters in use today typically include a resilient supporting mat that is wrapped around the periphery of the honeycomb. This resilient material, which distributes any compressive forces uniformly on the ceramic, typically expands as the temperature increases. This being the case, the compressive supporting pressure on the honeycomb therefore increases at elevated temperatures, and in some degree compensates for the thermal expansion of the outer metal shell. Since the metal shell expands more than the enclosed ceramic honeycomb, this mat expansion with temperature rise, prevents the honeycomb from becoming loose in the shell.
There are known in the art various techniques for assembling the mat and ceramic monolith into a can to make catalytic converters as described above. In general, the existing techniques can be divided into two groups: (1) those processes where a pre-determined gap between the ceramic monolith and the metal shell is maintained during assembly (e.g., called stuff mounting or clam shell mounting); and (2) those processes where the pressure between the metal shell and the ceramic monolith can be adjusted during assembly (e.g., shoe-box and tourniquet mounting).
Stuff mounting techniques involve initially wrapping the substrate in a resilient mat and thereafter inserting the wrapped substrate into a conical device that compresses the mat around the substrate as both are pushed into the cone. The wrapped substrate is then pushed from the compression cone into a cylindrical tube that serves as the converter container or shell (see, for example U.S. Pat. No. 4,093,423 to Neumann). Clam shell canning involves the utilization of two metal shell halves which are closed around a mat-wrapped honeycomb and thereafter welded together; (see for example U.S. Pat. No. 5,273,724 to Bos).
The tourniquet method involves forming a rectangular flat sheet metal piece into a cylindrical shell around a mat-wrapped honeycomb, the combined assembly then being pulled together to overlap the sheet edges and develop a predetermined level of mat compression around the honeycomb. Thereafter, the lap joint is welded together to maintain the level of compression (see for Example U.S. Pat No. 5,082,479 to Miller).
All of the prior art methods of assembling catalytic converters discussed above all involve subjecting the ceramic substrate to substantial and sometime uneven compressive forces during catalytic converter assembly, as the supporting mat is compressed against the substrate during the mat pre-compression step of canning. In fact, the compressive stresses reached during canning can significantly exceed those encountered by the substrate in actual use, due to factors such as the room temperature visco-elastic characteristics of the mat material.
Damage to the catalyst substrate resulting from applied stress during canning has not been a problem for conventional catalyst substrates due to the relatively robust honeycomb structural designs that have been used. However, demands for improved catalyst performance have resulted in increasing demand for thin-wall and ultra-thin-wall catalyst substrates (e.g., substrates with honeycomb channel wall thicknesses approximating 0.004 inches [100 micrometers] or less, typically 25-100 micrometers).
With these developments therefore a need has arisen for an improved canning process that would reduce canning stress on honeycomb catalyst supports.
The present invention involves the method for mounting a honeycomb catalyst support within a metal shell wherein the support is entirely shielded from the forces of mat pre-compression. In accordance with that method the initial compression of the supporting mat is accomplished by pre-compressing the mat against the shell utilizing tooling designed for mat pre-compression, and then axially inserting the honeycomb substrate into opening formed by the shell and precompressed mat. The insertion step is generally carried out with no further compression of the precompressed mat.
The advantage of this approach is that it avoids the stresses inherent in prior art canning methods wherein mat compression is achieved by compressing the mat with the ceramic substrate itself. In other words, the instant assembly method comprises providing an (internally) mat layered metal shell, compressing the mat layer against the metal shell, and inserting the honeycomb substrate into the mat-layered shell. The compressive stresses then applied to the substrate by the precompressed mat in the course of compression release during or after substrate insertion are far less intense that those developed during the mat compression in most conventional canning procedures.
In general, the basic method of assembling these catalytic converters comprises the following steps: (1) providing an open-ended one piece metal shell; (2) positioning a layer of resilient mat material on the inside surface of the metal shell to form an encircling mat layer; (3) compressing the encircling mat layer; and, (4) inserting the ceramic substrate into the metal shell while retaining the encircling mat layer on the inside surface of the metal shell. In preferred embodiments mat layer compression is accomplished by passing a tapered, closely fitting arbor member into and through the mat-lined shell, followed immediately by the insertion of the ceramic substrate. However, some modification of these basic steps is required where the outlet end of the metal shell is constricted or blocked.
Canning substrates into end-constricted or end-blocked enclosures is required to be carried out for a number of common catalytic converter designs. For example, it is frequently advantageous to carry out the canning process using a shell that already incorporates a conical end element or section for the conveyance of exhaust gases into or out of the converter. Also common are catalytic converter designs wherein multiple ceramic substrates must be mounted in a single elongated metal shell. In either of these cases, the use of arbor tooling designed to pass completely through an open-ended shell for the purpose of mat pre-compression is not practical.
The present invention includes a process designed to overcome these limitations, by carrying out preliminary mat pre-compression and substrate insertion steps within an intermediate or so-called xe2x80x9cslavexe2x80x9d enclosure. More particularly, this catalytic converter assembly method comprises providing an open-ended slave enclosure having a slave open cross-section defined by an inside surface, that enclosure having an open cross-section corresponding substantially to the cross-section of a selected ceramic substrate as well as to the open cross-section of an end-blocked or end-constricted metal shell designed to provide the actual catalytic converter enclosure.
Proceeding with the canning process, a layer of resilient mat material is positioned against the inside surface of this slave enclosure to form an encircling mat layer and that layer is compressed against the inside surface to provide a precompressed layer for receiving the ceramic substrate. Without further mat compression, and prior to or concurrently with any mat de-compression, the ceramic substrate is then inserted into the mat-lined slave enclosure while maintaining the position of the mat layer within the enclosure.
Once the mat and substrate have been mounted in the slave enclosure, the open cross-section of that enclosure is aligned with the open cross-section of the metal converter shell and the substrate and mat are transferred as a unit into the latter. This transfer step may be carried out, for example, through the use of a pushing member that applies sliding force simultaneously to the substrate and mat. Transfer is most preferably accomplished without significant further compression of the encircling mat layer.
Using this slave enclosure and transfer method, no traversal of the converter shell by the compression tooling is required. Therefore, end blockages or constrictions in the metal shell, whether arising from the shell design or from one or more substrates previously mounted in the shell, are easily dealt with.